This invention concerns screening of objects from a water flow using a drum screen, and more particularly is concerned with an efficient flow pattern and arrangement of components in a drum screen system. The invention applies to wastewater treatment plants and other situations as well.
There are a number of situations in which a flow of water, especially water containing impurities, must be screened to remove relatively large particles or items prior to further steps. For example, in a wastewater treatment plant the first process applied to incoming wastewater typically involves screening. Refractory items, including plastics, and paper and other materials larger than the screen opening size are separated out since these materials typically cannot be digested. A typical range of screen size for wastewater plants is 0.5 mm to 10 mm. Screen types have included belt screens, spiral screens and drum screens, all of which include some means for removing the screened out items.
Drum screens in such applications are typically large, with diameters of 15 feet or 18 feet or even larger, such as 36 feet or greater. Large wastewater treatment plants, e.g. handling 150 million gallons per day, have employed drum screens of 36 feet diameter or greater. Many smaller plants have screened using drum screens in the range of about 14 feet to 26 feet diameter.
These typical drum screen systems the drums, which receive water from the side, have been positioned with the rotation axis parallel to the direction of flow in the channel. Thus, water enters the drum on a direct path axially into the drum, typically flowing into the lower half of the drum, flows through the screen and then continues in the same flow direction. Debris is trapped on the screen and removed out one side of the drum.
Such drum screen systems require special water flow channels, wide enough to contain the diameter of the drum. In many cases of existing plants that might advantageously be retrofitted with drum screens to replace another type of screen, the channels simply are not wide enough to accept drums of a large enough size to handle the flow. Even in a newly constructed plant or screening subsystem, considerable space is required for drum screens of adequate size, with the typical axial inflow pattern conventionally used. A more efficient drum screen system is needed.
U.S. Pat. No. 5,407,563 describes screening panel constructions for use in band screens and drum screens. FIG. 3 of that patent shows a schematic indication of a double-entry drum screen, the drum being of T-frame construction. The subject matter of the patent is the particular screen panel construction, and little description is given for FIG. 3, but the drawing has some relevance to the present invention.